A group of temperature-sensitive mutants, ts1, ts7, and ts11, of Moloney murine leukemia virus when injected into neonate CFW/D mice, rapidly and invariably induce fatal hind-limb paralysis. These paralytic mutants are also defective in the processing of the precursor of the envelope proteins, gPR80env, in the infected cell. In order to understand the molecular mechanism of the mutant-induced paralysis we propose specifically: (1) To determine the molecular basis of the mutation and whether paralytogenicity and inefficiency in the processing of gPr80env are due to the same mutation. Hybrid genomes will be constructed by "swapping" fragments between the molecularly cloned genomes of ts1 and wild type Mo-Mu1V. The infectious virus obtained from transfected cultures will be characterized with respect to the disease phenotype it produces, its temperature sensitivity, and its ability to process gPr80env. (2) To investigate whether the mutant has acquired neurotropism and if so to what extent it can replicate in the infected neurons. The binding characteristics of tsl and wt to spinal cord cells of CFW/D mice will be compared. Virus replication in infected neurons and muscle cells will be determined and viral RNA and protein synthesis will be examined using in situ hybridization and immunocytochemical analysis. (3) To determine what effect the virus and in particular gPr80env may have on neuromuscular transmission. Env precursor protein accumulation in the neurons will be analyzed by immuno-precipitation. In vitro electrophysiological experiments will be carried out to determine whether ts1 infection inhibits neuromuscular transmission. The studies proposed are significant for the light they may shed on retrovirus-induced paralysis (which is a model for virus-induced human neurological diseases) as well as for their relevance to enhancing our understanding of retrovirus replication.